In optical communications networks, optical communications modules, which include optical transmitter modules, optical receiver modules, and optical transceiver modules, are used to transmit and/or receive optical signals over optical fibers. In a transmit portion of such an optical communications module, a light source generates modulated optical signals that represent data, which are then coupled by an optics system into an end of an optical fiber for transmission over the optical fiber. The light source may be a light-emitting diode or a laser such as, for example, a Vertical Cavity Surface Emitting Laser (VCSEL) or an edge-emitting laser. In a receive portion of such a module, an optics system directs light propagating out of the end of an optical fiber onto an optical detector, which converts the optical energy into electrical energy. A optical detector is typically a semiconductor photodiode device, such as a PIN (p-type/intrinsic/n-type) photodiode.
An optical communications module is commonly assembled by mounting the optoelectronic device, i.e., laser or optical detector, on a circuit board, making electrical connections between electrical contacts of the optoelectronic device and electrical contacts of the circuit board, and mounting an optics system on the circuit board. During the assembly process, a vision-aided passive alignment system and method are used in conjunction with fiducial features located on the optoelectronic device, the circuit board and/or on the optics system to ensure that the optics system and the optoelectronic device are properly aligned with one another throughout the mounting process. If the optic system and the optoelectronic device are not properly aligned with one another at completion of the mounting process, the optical coupling efficiency of the assembled optical communications module may be inadequate.
FIGS. 1A-1C illustrate side plan views of an optics system 2 being mounted on a circuit board 3 on which an optoelectronic device 4 is mounted during various stages of the mounting process. These figures demonstrate an example of the manner in which one type of misalignment can occur between the optic system 2 and the optoelectronic device 4 during the mounting process, even when a passive alignment process is used to prevent misalignment. It is also known to use active alignment processes to prevent misalignment from occurring during the mounting process, but because the invention is directed to passive alignment systems and processes, active alignment processes and systems will not be discussed herein in the interest of brevity.
During the mounting process, an upward-looking camera (not shown) of a vision system (not shown) captures images of a lens 5 disposed on a lower surface 2a of the optics system 2 as the optics system 2 and/or the circuit board 3 is moved by a positioning system (not shown) in the X-, Y- and/or Z-dimensions of an X, Y, Z Cartesian coordinate system. A downward-looking camera (not shown) of the vision system captures images of the optoelectronic device 4 as the optics system 2 and/or the circuit board 3 is moved in the X-, Y- and/or Z-dimensions. The images that are captured by the cameras are processed by a computer (not shown) of the vision system. The computer calculates positional adjustments that need to be made to the relative positions and/or orientations of the optoelectronic device 4 and the optics system 2 to maintain their alignment or to realign them. The computer then causes corresponding control signals to be sent to the positioning system, which then adjusts the relative positions and/or orientations of the circuit board 3 and/or the optics system 2 to ensure that they remain in alignment during and through completion of the mounting process.
At the stage of the mounting process shown in FIG. 1A, end surfaces 2b and 2c of two standoffs 2d and 2e, respectively, of the optics system 2 are about to come into contact with an upper surface 3a of the circuit board 3. The center 5a of the lens 5 and the aperture 4a of the optoelectronic device 4 are aligned with each other along a Z-axis of the X, Y, Z Cartesian Coordinate system. It can be seen in FIG. 1A that the X-Y plane in which the end surfaces 2b and 2c of the standoffs 2d and 2e, respectively, lie are not parallel with the plane in which the upper surface 3a of the circuit board 3 lies. Ideally, these surfaces are parallel to one another and parallel to the X-Y plane at the instant in time when the end surfaces 2b and 2c of the standoffs 2d and 2e, respectively, make contact with the upper surface 3a of the circuit board 3. In other words, the end surfaces 2b and 2c of the standoffs 2d and 2e, respectively, should make contact with the upper surface 3a of the circuit board 3 at precisely the same instant in time.
In the situation shown in FIG. 1A, although the center 5a of the lens 5 and the aperture 4a of the optoelectronic device 4 are aligned along the Z-axis, the X-Y plane in which the surfaces 2b and 2c of the standoffs 2d and 2e, respectively, lie is tilted by a tilt angle relative to the plane in which the upper surface 3a of the circuit board 3 lies. The magnitude of this tilt angle corresponds to an amount of linear misalignment in the X-dimension between the optics system 2 and the optoelectronic device 4. The amount of linear misalignment in the X-dimension that is present may be so small that the computer of the vision system will process the images captured by the cameras and make an erroneous determination that the lens 5 is in precise alignment with the optoelectronic device 4. Consequently, the computer will cause the motion system to continue to create the relative movement in the Z-dimension that is needed to mount the optics system 2 on the circuit board 3. There may also be tilt misalignment relative to the X-Z plane resulting in linear misalignment in the Y-dimension, but for ease of illustration and in the interest of brevity, only tilt misalignment relative to the X-Y plane is described with reference to FIGS. 1A-1C.
As shown in FIG. 1B, because of the tilt misalignment, the end surface 2b of standoff 2d comes into contact with the upper surface 3a of the circuit board 3 before the end surface 2c of standoff 2e comes into contact with the upper surface 3a of the circuit board 3. Subsequently, the end surface 2c of standoff 2e comes into contact with the upper surface 3a of the circuit board 3, as shown in FIG. 1C. Thus, the location where the upper surface 3a comes into contact with the end surface 2b of the standoff 2d acts as a pivot point. The result of the linear misalignment in the X-dimension is that the center 5a of lens 5 is not aligned with the aperture 4a of the optoelectronic device 4 in the Z-dimension in the final mounted configuration shown in FIG. 1C. In the example shown in FIGS. 1A-1C, the optics system 2 includes a 45° mirror 2f (FIG. 1C) that turns the optical path of the optics system 2 by an angle of 90°. The optical path portion 2g (FIG. 1C) should be coaxial with an optical axis of an optical fiber (not shown) that is inserted into a receptacle 2h (FIG. 1C) of the optics system 2. Because of the aforementioned misalignment, the optical path portion 2g will not be coaxial with the optical axis of the fiber, which results in optical coupling inefficiencies.
As the optics system 2 and the upper surface 3a of the circuit board 3 are moved into close proximity with one another, there will be an instant in time when the view from the upward-looking camera to the optics system 2 is occluded by the circuit board 3. After that instant in time, no real-time alignment adjustments are made due to the fact that the upward-looking camera can no longer obtain images of the lens 5 of the optics system. Therefore, the optics system 2 and the upper surface 3a of the circuit board 3 will be brought into contact with one another in accordance with the last alignment adjustment that was made using the last images from the downward- and upward-looking cameras. This can also result in the type of misalignment shown in FIG. 1C.
Accordingly, a need exists for a system and method for performing vision-aided passive alignment during the mounting process that ensure that the aforementioned misalignment problem does not occur.